Hans Kraus

Abstract:

Oxide crystal scintillators play a considerable role in fundamental and applied researches. However, working out of new generation of high-sensitivity equipment and new methods of research puts higher requirements. The ZnBO 4 (B = W, Mo) crystals were grown from charge in platinum crucibles with high frequency heating, using the Czochraiski method. The raw powder with optimum composition was prepared by solid phase high temperature synthesis using ZnO and BO3 (B = W,Mo) with 4-5N purity. Single crystals with sizes up to ∅ 50× 100 mm were grown and scintillation elements of various sizes and shapes (cylinders, rectangular and hexahedron prisms) were produced. High spectrometric characteristics were obtained for ZnWO4:R = 8-10% under excitation by 137Cs [Eγ= 662 keV), low radiation background (less than 0.2 mBq/kg) and low afterglow (0.002%, 20 ms after excitation). The obtained results demonstrate good prospects for ZnWO 4 and ZnM0O4 crystal scintillators for application in low-count rate experiments, searching for double beta decay processes, interaction with dark matter particles, and also studies of rare decay processes. The material has also a, good potential for application in modern tomography, scintillation bolometers and for other major researches using scintillators.

Abstract:

EURECA (European Underground Rare Event Calorimeter Array) is an astro-particle physics facility aiming to directly detect galactic dark matter. The Laboratoire Souterrain de Modane has been selected as host laboratory. The EURECA collaboration unites CRESST, EDELWEISS and the Spanish-French experiment ROSEBUD, thus concentrating and focussing effort on cryogenic detector research in Europe into a single facility. EURECA will use a target mass of up to one ton, enough to explore WIMP - nucleon scalar scattering cross sections in the region of 10 - 10 picobarn. A major advantage of EURECA is the planned use of more than just one target material (multi target experiment for WIMP identification). © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

Abstract:

CRESST-II, standing for Cryogenic Rare Events Search with Superconducting Thermometers phase II, is an experiment searching for Dark Matter. In the LNGS facility in Gran Sasso, Italy, a cryogenic detector setup is operated in order to detect WIMPs by elastic scattering off nuclei, generating phononic lattice excitations and scintillation light. The thermometers used in the experiment consist of a tungsten thin-film structure evaporated onto the CaW04 absorber crystal. The process of evaporation causes a decrease in the scintillation light output. This, together with the need of a big-scale detector production for the upcoming EURECA experiment lead to investigations for producing thermometers on smaller crystals which are glued onto the absorber crystal. In our Run 31 we tested composite detectors for the first time in the Gran Sasso setup. They seem to produce higher light yields as hoped and could provide an additional time based discrimination mechanism for low light yield clamp events. © 2009 American Institute of Physics.

Abstract:

The CRESST experiment aims for a detection of dark matter in the form of WIMPs. These particles are expected to scatter elastically off the nuclei of a target material, thereby depositing energy on the recoiling nucleus. CRESST uses scintillating CaW04 crystals as such a target. The energy deposited by an interacting particle is primarily converted to phonons which are detected by transition edge sensors. In addition, a small fraction of the interaction energy is emitted from the crystals in the form of scintillation light which is measured in coincidence with the phonon signal by a separate cryogenic light detector for each target crystal. The ratio of light to phonon energy permits the discrimination between the nuclear recoils expected from WIMPs and events from radioactive backgrounds which primarily lead to electron recoils. CRESST has shown the success of this method in a commissioning run in 2007 and, since then, further investigated possibilities for an even better suppression of backgrounds. Here, we report on a new class of background events observed in the course of this work. The consequences of this observation are discussed and we present the current status of the experiment. © 2009 American Institute of Physics.

Abstract:

The behaviour of the crystal structure of ZnWO4 was investigated by means of synchrotron and neutron powder diffraction in the range of 3-300K. Thermal analysis showed the sample's melting around 1486K upon heating and subsequent solidification at 1442K upon cooling. Therefore, the structure was also investigated at 1423K by means of neutron diffraction. It is found that the compound adopts the wolframite structure type over the whole temperature range investigated. The lattice parameters and volume of ZnWO4 at low temperatures were parametrized on the basis of the first order Grüneisen approximation and a Debye model for an internal energy. The expansivities along the a-and b-axes adopt similar values and saturate close to 8 × 10 -6K-1, whereas the expansion along the c-axis is much smaller and shows no saturation up to 300K. The minimum expansivity corresponds to the direction close to the c-axis where edge-sharing linkages of octahedra occur. © 2009 IOP Publishing Ltd.